Apolipoprotein (apo) E and apoA-1 play several critical roles in modulating and in directing plasma lipoprotein metabolism. Dysfunction by either protein leads to increased atherosclerosis and ultimately to heart disease and stroke. In addition, apoE4 is an established risk factor for neurodegeneration and poor outcome from head injury and stroke. Despite the critical roles that apoE and apoA-1 play in normal physiology, detailed structural information is not available for either of these proteins in their natural lipid-bound state. This proposal is based on our recent technological breakthrough of crystalling apolipoprotein-phospholipid complexes and is designed to provide, for the first time, detailed structural information on apoE and apoA-I bound to lipid. Three specific aims focus on relevant issues and questions related to the structure and function of apoE and apoA-I. 1. The structures of apoE-dimyristoylphosphatidylcholine (DMPC) complexes will be determined to ascertain how lipid association influences apoE function. 2. The hypothesis that apoA-1 adopts the so-called "belt model" conformation on a phospholipid disc will be tested and structural details on the interaction of apoA-1 amphipathic helices with lipid and insights into LCAT activation will be provided. 3. To analyze the effect of cholesterol incorporation into phospholipid discoidal particles on the structure of apoE and apoA-1. These studies will provide novel structural information on the effect of lipid association on the structure and function of apoE and apoA-1 and provide, for the first time, insights into how these proteins function in their natural lipid-associated state.